This invention relates to a method for the recovery of alumina from pulverized coal fly ash. More specifically, this invention relates to an improvement in the lime-sinter process for the recovery of alumina from pulverized coal fly ash.
Fly ash is being produced in increasing quantities as the result of the expanding use of pulverized coal to produce electricity. Fly ash is formed from the inorganic constituents present in the coal, and is collected from the flue gas by mechanical separation and/or electrostatic precipitation. The primary constituents of the ash are the oxides of silicon, aluminum, iron and calcium. In addition, trace elements of selenium, arsenic and other heavy metals are present.
The treatment of fly ash to recover its constituent metals has been proposed as an alternative to disposal, since, for example, a typical fly ash may contain up to fourteen percent aluminum by weight.
Several processes have been proposed for the recovery of aluminum from fly ash. One such process is disclosed in U.S. Pat. No. 4,159,310 to Reynolds et al, dated June 26, 1979. In this process, the fly ash is contacted with chlorine in an oxidizing atmosphere to selectively chlorinate and vaporize iron chloride. The residue is then chlorinated in a reducing atmosphere, vaporized and selectively condensed to separate and recover the vaporized chlorides of aluminum silicon and other metals.
Another process, described in U.S. Pat. No. 4,130,627 to Russ et al, dated Dec. 19, 1978, heats the fly ash in an aqueous alkali solution to form an aqueous blend. The blend is filtered to recover an alkaline solid which is then contacted with hydrochloric or hydrofluoric acid to form soluble salts of iron, aluminum and other metals, which can be recovered and separated by selective percipitation.
In the lime-soda sinter process, a mixture of fly ash, calcium carbonate and sodium carbonate are sintered to form a clinker containing soluble sodium and calcium aluminates. The clinker is reduced to powder and the soluble compounds dissolved in a dilute solution of sodium carbonate.
While these processes are satisfactory for producing an aluminum compound suitable for commercial sale, the processes are complex and/or utilize reactants which are expensive, thus substantially increasing the cost of the resulting product.
In a less costly process, about one part fly ash and two parts calcium carbonate are sintered at 1400.degree. C. to form a clinker containing a calcium aluminate compound, which is then dissolved in a dilute sodium carbonate solution. Although the reactants are inexpensive, the process recovers only 50 to 60% of the available aluminum and requires high sinter temperatures which result in high energy and equipment costs.